1) Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a liquid crystal display device free from display area restrictions and able to be operated by a matrix drive.
2) Description of the Related Arts
It is known that a liquid crystal display (LCD) device can be operated under a low drive voltage at a low power consumption, and be constructed as a compact and thin device. In particular, a TN type LCD can be operated under a low voltage at a low power consumption, and thus is widely used in various fields, for example, watches and desk-type electronic calculators.
The growing popularity of word processors, personal computers, and other data processing devices has led to a demand for portable, small, and thin devices, and to this end, LCD devices are replacing the CRT (cathode ray tube) device as the display element in those machines. With the LCD device, it is possible to reproduce the image of Chinese characters on the screens of these machines, since a great number of picture elements can be used in the LCD device in comparison with the number of picture elements used in the display device used for watches and desk calculators. Also, the LCD element is operated by a matrix display drive in which picture element electrodes are connected to signal lines in an X-Y matrix form. In this type of LCD device, the picture element electrodes corresponding to the picture elements are not independent from each other, and thus when a predetermined voltage is applied to one picture element electrode, the adjacent picture element electrodes are also operated by this voltage and display an image, i.e., "cross-talk" occurs; "cross-talk" occurs between the one picture element electrode and adjacent picture element electrode.
To eliminate this cross-talk, it s known to use a non-linear element consisting of a diode, for example, metal-insulator-metal diode (MIM) or thin film transistor (TFT) for each picture element electrode, but it is very difficult to provide and arrange a large number, for example, several thousands to several hundreds of thousands, of diodes or thin membrane transistors corresponding to a large number of picture elements, all of which must have uniform properties and be free from defects, and thus there is an urgent need for the development of non-linear elements which can be easily provided with a uniform quality and will allow the use of an LCD device with a large display area. In connection with the above, a new type of liquid crystal material usable for a display in a large display area has been developed through a new technology involving an NCAP (nematic curvilinear aligned phase) type liquid crystal material. With this technology, it is now possible to easily control the thickness of the liquid crystal material layer in the display device, and therefore, the NCAP type liquid crystal material can be formed as a layer having a large surface area, exhibits a very quick response, and allows the resultant display device to be given a wide angle of view without the use of a polarizer plate, and thus can be beneficially utilized to provide an LCD device having a large display surface area.
Nevertheless, the NCAP type liquid crystal material is disadvantageous in that this liquid crystal material requires a drive voltage of several tens to several hundreds of volts higher than that of TN type liquid crystal material, e.g., 5 volts or less, and therefore, a non-linear element having a higher drive voltage than that of conventional non-linear elements must be used for the NCAP type LCD device.
Also, the NCAP type liquid crystal material is disadvantageous in that the voltage-contrast property is not sufficient to produce a really sharp image, and thus when operated by a multiplex drive (simple matrix drive), the resultant display is not absolutely clear.
Further, since the NCAP type liquid crystal material needs a high drive voltage, it is difficult to utilize TFT or MIM having a low drive voltage as a non-linear element, and accordingly, it is considered by persons skilled in the art that NCAP type liquid crystal devices can not be operated by a matrix drive.